Columns not belonging in a diaphragm
In the case of storey columns not belonging in a diaphragm, the stress induced by accidental eccentricities may be obtained approximately, assuming that the diaphragmatic behaviour is approximated by the grid of beams around the columns.
Vertical component of seismic action [EC8, §188.8.131.52.2]
If avg≤0.25g the vertical component of the seismic action may be ignored. Substituting the values ofagR=0.16g for seismic zone 1 and γI=1.40 for importance class IV the expression avg=0.9ag=0.9 · γI · agRyields 0.9 · γI · 0.16g=γI · 0.144g → avg=0.20g <0.25g, therefore the vertical seismic component check may be omitted.
In case however where avg>0.25g, the vertical component of the seismic action should be taken into account only in the cases listed below:
- for horizontal or nearly horizontal structural members spanning 20 m or more
- for horizontal or nearly horizontal cantilever components longer than 5 m
- for horizontal or nearly horizontal pre-stressed components
- for beams supporting planted columns
- in base-isolated structures
In these special cases simplified local models may be used whose results may be taken into account only for the special elements.
In the case of planted columns, an extra gravity load may be added to the column with value being equal to the seismic force FV obtained from the design load G+ψ2 · Q of the column multiplied by thevertical seismic acceleration aV, i.e Fv=(aV/g) · (G+ψ 2 · Q). In this case, all the necessary combinations of horizontal and vertical loads are automatically considered for the whole structure.
To simplify the calculations as well as for safety reasons, the vertical acceleration may be taken equal to the maximum value that may occur (see §6.1.7).
Example 6.6.2: Consider building with a planted column, in a seismic zone with agR=0.24 and importance class III (γI=1.2):
avg=0.9 · γI · agR=0.9 · 1.2 · 0.24 · g=0.26g, thus the vertical component of the seismic action should be taken into account at the planted column region.
The maximum acceleration developed may be calculated by means of §6.1.7:
aV,max=1.5 · γI · agR =1.5 · 1.2 · 0.24g=0.432g.
The dead load G of the planted column may be replaced by another load being equal to FV=G+0.432 · (G+ψ2 · Q).
If the magnitude of the additional force FV of the planted column is significant, the apparent favourable influence of this probable additional force should be taken into account on the moments (therefore also on the reinforcement) of the adjacent beams, because it is actually a quasi-live load and not a dead one.